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WASHINGTON — North Korea confirmed on Tuesday that it had conducted its third, long-threatened nuclear test, according to the official KCNA news service, posing a new challenge for the Obama administration in its effort to keep the country from becoming a full-fledged nuclear power.

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The KCNA said it used a “miniaturized and lighter nuclear device with greater explosive force than previously” and that the test “did not pose any negative impact on the surrounding ecological environment.”

Many nations initially detected the test as seismic activity centered near the same location where the North conducted tests in 2006 and 2009. The United States Geological Survey said it was only a kilometer underground, an indication consistent with a nuclear blast. And in Vienna, the organization that monitors the Comprehensive Nuclear Test Ban Treaty said that tremor had “clear explosionlike characteristics.”

Preliminary estimates suggested a test far larger than the previous two conducted by the North, though probably less powerful than the first bomb the United States dropped on Japan, in Hiroshima, in 1945.

If confirmed, the test would be the first under the country’s new leader, Kim Jong-un, and an open act of defiance to the Chinese, who urged the young leader not to risk open confrontation by setting off the weapon. In the past few days a Chinese newspaper that is often reflective of the government’s thinking said the North must “pay a heavy price” if it proceeded with the test. But it was unclear how China would act at the United Nations Security Council, which was heading into emergency session as news of the suspected blast played out.

The Obama administration has already threatened to take additional action to penalize the North if it conducts a test, through the United Nations. But the fact is that there are few sanctions left to apply against the most unpredictable country in Asia. The only penalty that would truly hurt the North would be a cutoff of oil and other aid from China. And until now, despite issuing warnings, the Chinese have feared instability and chaos in the North more than its growing nuclear and missile capability, and the Chinese leadership has refused to participate in sanctions.

Mr. Kim, believed to be about 29, appears to be betting that even a third test would not change the Chinese calculus.

The apparent test set off a scramble among Washington’s Asian allies to assess what the North Koreans had done.

The United States sent aloft aircraft equipped with delicate sensors that may, depending on the winds, be able to determine whether it was a plutonium or uranium weapon. The Japanese defense minister, Itsunori Onodera, said Japan had ordered the dispatch of an Air Self-Defense Force jet to monitor for radioactivity in Japanese airspace.

Japan’s new prime minister, Shinzo Abe, told Parliament that “based on precedents, Japan believes that this quake was triggered by a North Korean nuclear test,” and said the country was considering “its own actions, including sanctions, to resolve this and other issues.”

But the threat may be largely empty, because trade is limited and the United States and its allies have refrained from a naval blockade of North Korea or other steps that could revive open conflict, which has been avoided on the Korean Peninsula since an armistice was declared 60 years ago.

It may take days or weeks to determine if the test, if that is what it proves to be, was successful. American officials will also be looking for signs of whether the North, for the first time, conducted a test of a uranium weapon, based on a uranium enrichment capability it has been pursuing for a decade. The past two tests used plutonium, reprocessed from one of the country’s now-defunct nuclear reactors. While the country has only enough plutonium for a half-dozen or so bombs, it can produce enriched uranium well into the future.

No country is more interested in the results of the North’s nuclear program, or the Western reaction, than Iran, which is pursuing its own uranium enrichment program. The two countries have long cooperated on missile technology, and many intelligence officials believe they share nuclear knowledge as well, though so far there is no hard evidence.

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The Iranians are also pursuing uranium enrichment, and one senior American official said two weeks ago that “it’s very possible that the North Koreans are testing for two countries.” Some believe that the country may have been planning two simultaneous tests, but it could take time to sort out the data.

The timing of the apparent test was critical. It came just as a transition of power is about to take place in South Korea, and the North detested the South’s departing president, the hard-line Lee Myung-bak. By conducting a test just before he leaves office, the North could have been both sending a message and giving his successor, Park Geun-Hye, the chance to restore relations after the breach a test will undoubtedly cause.

There had also been predictions that North Korea might hold a test on Tuesday because it is the day of President Obama’s State of the Union address.

Western officials considered the country’s first nuclear test, in 2006, a failure, but the next one, in 2009, was judged more successful. It may take outside experts days or weeks to determine if the latest blast moved the program to a “higher level,” as Pyongyang recently promised.

While intelligence officials in Washington and Seoul are jittery about the North’s progress, there is still no proof that it has yet mastered the difficult technology of miniaturizing bombs so they can be fitted to ballistic missiles. But arms experts declared a recent rocket launching a success, suggesting the country was making advances that could eventually allow it to lob a nuclear-tipped missile as far as the United States mainland.

The apparent nuclear test came just weeks after the United Nations Security Council unanimously passed a Washington-led resolution calling for the tightening of sanctions against North Korea for that rocket launching, a violation of earlier resolutions prohibiting the country from testing ballistic missile technology.

Stung by the promise of stiffer sanctions, Pyongyang ratcheted up its threats, vowing to build its capacity to “target” the United States in its most explicit warnings yet. The statement last month, one in a series of threatening statements over several days, said the country planned to test more long-range rockets (“one after another”) and to conduct a nuclear test, despite Washington’s warning that such actions would lead to more penalties for the impoverished country.

Pyongyang has often lashed out when it felt ignored, especially by the United States. It was unclear if the untested Mr. Kim was following a pattern of behavior perfected by his father, the last North Korean leader, in which the North provoked the West and Seoul to win more badly needed aid as an inducement to draw it back to international negotiations on its weapons programs.

Analysts suspect that Mr. Kim, in the face of more sanctions, might have felt a more urgent need to assert his standing among his people, who continue to suffer crippling food shortages they are told is the price of developing a costly and credible deterrence. He also might have needed to improve his standing with the military, which has been considered crucial to keeping the Kims in power, analysts said.

David E. Sanger reported from Washington, and Choe Sang-hun reported from Seoul, South Korea. Jane Perlez contributed reporting from Beijing and Hiroko Tabuchi from Tokyo.